The present invention relates to a multiple-layered bucket cover for turbines and particularly relates to a multi-layered bucket cover having an intermediate perforated layer and methods of assembly.
A turbine rotor typically mounts a plurality of circumferentially spaced, generally radially extending airfoils or buckets. Covers are conventionally provided on the tips of the buckets, the covers forming a 360xc2x0 annulus about the buckets with a small clearance between the outer surface of the cover and the surrounding shroud.
There are a number of different structures and methods for securing covers to the tips of the buckets. One such structure is known as a button tenon/cover configuration. In that construction, one or more tenons projecting generally radially from each bucket pass through corresponding openings in the cover and are peened, preferably automatically, along the outer surface of the cover. This button tenon/cover configuration provides substantial pull strength, i.e., sufficient structural integrity between the cover and bucket, to preclude removal of the cover from the end of the bucket under centrifugal forces. The button tenon/cover configuration, however, does not provide adequate cover sealing. That is, the buttons form a series of radially outward projections along the outer surface of the cover, necessitating increased clearance between the rotating component, i.e., the covers, and the surrounding stationary component, i.e., a shroud, thereby increasing tip leakage losses. The button tenon/cover configuration, however, has the advantage of enabling the peening operation to be performed automatically.
In another configuration, known as a xe2x80x9cfoxholexe2x80x9d tenon/cover configuration, the tenon on the bucket is recessed from the outer opening of the cover. Because of the absence of buttons projecting beyond the outer surface of the cover, the xe2x80x9cfoxholexe2x80x9d configuration enables tighter tip clearance with the surrounding stationary component, affording improved cover sealing and reduced tip leakage losses. However, foxhole tenon/cover configurations require a manual peening process to secure the covers to the buckets. This process is physically laborious and costly. Accordingly, there is a need to provide a bucket cover which affords sufficient pull strength and which can be both automatically peened and provide adequate cover sealing to minimize tip leakage losses.
In accordance with a preferred embodiment of the present invention, there is provided a bucket cover formed of multiple layers or elements in an arcuate configuration spanning the outer tip of the buckets. The bucket cover is provided in multiple arcuate segments forming a complete annulus about the periphery of the rotor, each segment comprised of multiple elements. Preferably, each bucket cover segment includes an inner element, an outer element and intermediate element for disposition between the inner and outer elements. The segments, and hence the elements, may span three or more buckets and are fitted on tenons of the buckets.
Particularly, the elements are provided with circumferentially spaced openings in registration with one another for receiving the tenons on the ends of the buckets. It will be appreciated that the tenons have a reduced profile as compared with the airfoil profile of the buckets. A radius or chamfer is provided between the reduced profile of the tenon and the airfoil profile at the tip of the bucket. Each opening through the inner element has a radial inwardly increasing chamfer to overlie the radiussed portion at the juncture between the tenon and the bucket airfoil. The intermediate element has openings corresponding to the profile of the tenon. The outer element has openings having a radially outwardly directed chamfer. The inner, intermediate and outer elements are disposed sequentially on the buckets with the openings receiving the tenons. Projecting ends of the tenons may then be peened, preferably automatically, to secure the elements to the buckets. Any excess material of the tenons is removed, for example, by machining to provide a smooth, continuous surface along the outer circumference of the cover. In this manner, tight clearances can be maintained between the cover and the surrounding stationary component.
To provide the necessary pull strength, cavities are provided in the intermediate elements between adjacent tenons. The cavities are void of material but are closed by end walls of the intermediate element facing upstream and downstream directions of hot gas flow through the turbine and bounded by the outer and inner surfaces of the inner and outer elements, respectively. In this manner, a considerable weight reduction in the cover is effected and hence any tendency of the covers for displacement radially outwardly due to centrifugal force is minimized and significantly reduces the pull strength. While the cover elements can be radially aligned with one another at their circumferentially adjacent joints, the elements may also be staggered relative to one another such that the joints between circumferential adjacent elements are misaligned or staggered in a circumferential direction relative to one another.
By utilizing the foregoing described configuration, a flush tenon/cover configuration having adequate pull strength is provided which advantageously can be formed using automatic peening machinery. Simultaneously, cover sealing is improved by tightening clearances between the cover annulus and the surrounding shroud.
In a preferred embodiment according to the present invention, there is provided a cover for a turbine bucket having a tenon adjacent a tip of the bucket comprising discrete inner and outer arcuate cover elements and an intermediate arcuate cover element therebetween, the elements having generally radially aligned openings for receiving the bucket tenon, the outer element having a radially outward chamfer for receiving peened bucket tenon material to retain the elements on the bucket.
In a further preferred embodiment according to the present invention, there is provided a rotatable component for a turbine comprising a plurality of circumferentially spaced buckets rotatable about an axis and terminating in radially outwardly extending tenons, a cover for the buckets including a plurality of discrete arcuate cover segments, each segment having inner, outer and intermediate arcuately extending elements, the elements having generally radially aligned openings at circumferentially spaced locations along the segments for receiving tenons, the tenons being peened to secure the elements on the buckets and forming a generally smooth continuous outer surface with an outer surface of the outer element.
In a further preferred embodiment according to the present invention, there is provided a method of assembling covers on buckets of a rotary component of a turbine, including the steps of providing inner, outer and intermediate arcuate cover elements having openings therethrough for receiving tenons formed on the ends of the buckets, locating the inner, intermediate and outer cover elements in succession on the tenons of the buckets with ends of the tenons projecting from the outer element, peening projecting ends of the tenons to secure the elements to the buckets and providing a smooth continuous arcuate surface along the outer surface of the cover including along the peened ends of the tenons.